I’d like to get the ball rolling with this thread, and post some initial information that might be helpful. If you know about this subject it would be great if you could add your own suggestions or information! If you have questions, please ask here and I’ll do my best to answer them. I’ve been very enthusiastic about VR for several years and have used a number of different systems.
The following 6-minute video is a good starting point on the state of play with VR in 2019: Why now is the best time to invest in VR
In Andrew’s answer he mentioned Google Cardboard; you can find out more about that here. If you go that route, be aware that this is the very lowest manifestation of virtual reality, though it will give you a bit of an idea of what VR is like, and it is also very affordable if you have a smartphone. (Just know that higher-end systems are going to provide an immensely richer experience.)
If you have never tried VR, I highly recommend seeing if there is anywhere you can get a demonstration: Microsoft stores usually have demos of their headsets; many urban centers have VR meetups (search on Meetup.com) – typically full of people happy to offer demos; many public libraries are starting to offer VR equipment for use by the public. I went to demos as often as I could until I could afford to get VR equipment of my own.
If you have a high-end Samsung smartphone, Gear VR is a great entry-level system. I was very happy with my Gear VR for two years while I saved up enough to get my current high-end PC-based VR system. Certain other high-end smartphones work with Google Daydream VR, another affordable entry-level option if you already have a compatible smartphone. (In addition to a compatible phone, Gear or Daydream costs about $100.)
If you already have a Playstation 4, PlayStation VR (PSVR) is another great option. It will cost a few hundred dollars. I haven’t tried PSVR, but I hear great things about it. It has a lot of interesting software (including the forthcoming Dreams by Media Molecule, which is going to be a PSVR exclusive.
Oculus Go is a standalone VR device that would make a fantastic starter system. It’s like an optimized Gear VR, but self-contained and costing $200.
Oculus Quest is a higher-end standalone VR system that is coming sometime in spring 2019 (i.e. soon!). It will cost $400 and will provide a significantly higher-level VR experience compared to the Go. (I can explain more about that if anyone is curious, which would involve explaining the terms “3DOF” and “6DOF” – for now just know that 6DOF is much more immersive/embodied than 3DOF.) Even though I have a desktop PC system which has a lot more power, I am seriously considering getting a Quest for various reasons.